[FEATURE] Delegate builder methods in composition of classes

[ilia1243]

Describe the feature

Consider two classes with @Builder, one is a field of another. If methods of outer builder could "delegate" to methods of inner builder, it would be possible to build the whole composition in a simpler way.

@Builder
public class Inner {
    private String innerField;
}

@Builder
public class Outer {
    // This tells Lombok to use InnerBuilder inside OuterBuilder
    @BuilderDelegate
    private Inner inner;
    private String outerField;
}

Example usage:

Outer outer = Outer.builder()
            .outerField("outerField")
            .innerField("innerField")
            .build();

Existing way to do the same

        Outer outer = Outer.builder()
                .inner(Inner.builder()
                        .innerField("innerField")
                        .build())
                .outerField("outerField")
                .build();

This can already be achieved by customizing the OuterBuilder with some boiler-plate code:

@Builder
public class Outer {
    private Inner inner;
    private String outerField;

    public static class OuterBuilder {
        private final Inner.InnerBuilder inner = Inner.builder();

        private OuterBuilder inner(Inner.InnerBuilder inner) {
            return null;
        }

        public OuterBuilder innerField(String innerField) {
            inner.innerField(innerField);
            return this;
        }

        public Outer build() {
            return new Outer(this.inner.build(), this.outerField);
        }
    }
}

Related question with the same goal but different implementation https://stackoverflow.com/questions/54309139/delegate-and-builder-combination-in-lombok